Optical chemical sensors can be applied to determine concentrations of certain gases. One application is for measuring concentrations of various gases in an internal combustion engine's exhaust gas stream. One reason to do this is that environmentally driven emissions legislation is regularly being changed to require lower and lower emissions of pollutants from vehicles. In particular, The California Air Resources Board (CARB) is leading an effort with their OBDII (On-Board Diagnostics II) requirement. OBDII requires detection of emitted hydrocarbons in the 100 PPM (parts per million) range.
There are many design challenges to achieve a robust sensor that can measure hydrocarbon emissions while surviving an extreme environment found in vehicular applications. This sensor must not only achieve a sensitivity required to measure hydrocarbons in the 100 PPM range, but also operate in an exhaust gas temperature that can range from 400.degree. C. to over 1,000.degree. C., have a resolution of .+-.50 PPM, survive extreme levels of vibration, survive in an acid environment on an automotive exhaust system, and operate while exposed to significant electromagnetic interference (EMI).
Contemporary electrical-circuit based hydrocarbon sensors include a hydrocarbon sensor based on a wire that has a catalyst impregnated bead of ceramic disposed on it. When the wire is exposed to a hydrocarbon, the catalyst burns the hydrocarbon, the wire gets hot, and the resistance of the wire changes. This scheme is not sensitive enough or robust enough to vibration, and furthermore has poor repeatability.
Other electrical-circuit based hydrocarbon sensors include silicon based sensors. These silicon based sensors cannot operate at the elevated temperatures of the exhaust gas stream and therefore apply complex cooling means to maintain a relatively low temperature at the sensor so that it will survive. Being electrical-circuit based these sensors are also susceptible to mechanical failure of electrical interconnections due to vibration, and also to EMI exposure.
One prior art optical scheme absorbs hydrocarbons into a chemically selective cladding disposed in a core structure. When exposed to light the presence of the hydrocarbons causes some of the light in the core to be absorbed in the cladding layer. A sensed reduction in light output is then used as a measure of hydrocarbon concentration. This scheme lacks sufficient selectivity, and speed of response.
What is needed is an improved hydrocarbon sensor that operate in the extreme temperature range of an automotive exhaust gas system, have a resolution of .+-.50 PPM, survive extreme levels of vibration, survive in an acid environment on an automotive exhaust system, and operate while exposed to significant electromagnetic interference (EMI).